fixed wireless broadband and home phone

My understanding is that with fixed wireless broadband the old copper wire lines are retained. Following installation of FW broadband my previous ADSL 2 service was discontinued which resulted in my old copper wire phone connection also going. My question is why the old lines cannot still be used for voice calls thus preserving my home phone and extensions and avoiding the need to set up VOIP base and extensions?

The plan for NBN installation & copper line decommissioning is made by the government and is implemented by NBN Co. Once NBN is active on certain areas the existing copper network will be switched off.